Jeff Thumma, training underneath a Sioux Falls lake, told members of Minnehaha County dive team waiting at the surface that he had a problem. Then he tugged on his tether line, four sharp tugs — the sign of an emergency.

But even as an extra diver plunged in to retrieve Thumma, the stream of bubbles stopped rising to the surface from where he floated, 15 feet below the surface.

"That means he's not breathing," said Pete Gannon, CEO of Dive Rescue International, a company that trains around 60 percent of public safety dive teams in the U.S.

It took just two minutes for the rescue diver to reach Thumma and get him to the surface — a "quite quick" time, Gannon said — but it was too late. After spending several days on life support, Thumma died Sunday evening in Sioux Falls.

Exactly how Thumma, a trained diver with more than 150 dives on his track record, came to die in a Thursday training accident may not be known for several weeks. But the 43-year-old Citibank employee and dive team volunteer's death has resonated both here in Sioux Falls and around the country.

"Any time you lose somebody that you're that close with, it's tough," said Doug Brockhouse, who served with Thumma on the county's dive team. "When you're down there diving, you put your life in the other divers' hands. An awful lot of trust goes on down there, and when you lose one of those people, that hurts."

Thumma had made many friends on the dive team with his intelligence and passion.

"Jeff was a very energetic young man," Brockhouse said. "He brought a lot of smiles to people's faces on our dive team."

Brockhouse said Thumma knew how to speak several languages and was always observing more experienced members of the dive team to learn from what they did — though he was hardly a novice himself.

"He was always willing to go grab the extra piece of equipment and make sure you had what you need," Brockhouse said.

Thumma had joined the dive team in 2010 and racked up around 40 dives with the volunteer squad in the past four years.

None of those dives were in the Family Park lake where Thumma's accident occurred, but most were in similar situations — cold water with little or no visibility.

"The majority of the time you can't see anything," Gannon said, about rescue diving in general. "It's a pleasure if you can see two feet."

In Thursday's exercise, dive team members were practicing a standard search pattern. Thumma floated about 15 feet down, at the center of a search area. Another dive team member, tied to Thumma with a rope, swam in slowly expanding circles around him, searching the bottom.

Thumma had been under water for about 12 minutes before running into trouble. He dove with a full oxygen tank that should have lasted an hour and a half at his depth.

He was linked to the surface with the tether, and with a comm unit wired into his suit. He used that to alert the surface shortly before his emergency pulls on the rope.

When the rescue diver got Thumma to the surface, he was unresponsive, and was rushed to the hospital.

Officials kept Thumma's identity and condition secret for the next several days, as he lay on life support and his family rushed to Sioux Falls.

"The family is from out of town. We just needed an opportunity to talk to them and work that over," said Lynn DeYoung, Minnehaha County's emergency manager. "Based on their request, we held off (on releasing Thumma's name) until after they knew what the final outcome would be."

Thumma's death was the first in the four-decade history of Minnehaha County's dive team, and it's still unclear what happened.

Nationally, many of the deaths and near-deaths associated with public safety dive teams are caused by medical issues.

"It's not one thing. It's many things," said Gannon. "Sometimes lack of training. Sometimes diver error. Sometimes entanglement. And a lot of times lately it's medical. (A diver) has a heart attack under water, or a seizure under water."

DeYoung said the dive team can't rule anything out yet.

"Like a lot of accidents, there's a lot of small things that by themselves wouldn't cause a problem, but you add three or four things together and it may cause a problem," he said.

One reason medical issues may be the biggest cause of death for dive teams nationwide is because training — like the one Minnehaha County's team was doing last week — have reduced other, more preventable accidents.

"Unfortunately, most divers die in training, because that's probably what we do the most of," Gannon said. "It's dangerous training. There's no way to not do it. Lack of training also kills divers."

In a typical year, two to three public safety divers will die every year in diving accidents, Gannon said. There's another seven to 10 "near-misses."

It's not a coincidence, though, that medical issues sometimes emerge when divers are deep under water.

"The type of diving we do is really stressful," Gannon said. "That's why there's medical problems. Guy's heart rate gets going high, his blood pressure gets going high. There's a lot of things that can happen. And most of the time, we're out there looking for a body. That adds to the stress, too."

DeYoung said the Minnehaha County dive team doesn't require physicals from its members.

"We simply ask them are they in good physical condition. They check that off," he said. "Diving's a pretty demanding thing. We know if they're certified already to dive that they most likely passed that with their physicians already."

The Sioux Falls Police Department is leading the investigation of Thumma's death. But it will likely require analysis of Thumma's blood to determine exactly how and why he died.

"There's certain things to look for that would tell us if it's a diving injury or if it's a medical problem," Gannon said. "There's not much we can do at this point until we know the autopsy."

Now the Minnehaha County dive team has to deal with the aftermath of their friend and colleague's death.

"It's not something that's going to go away quickly for that team," Gannon said. "That team's going to hurt for a long time."

Brockhouse said the team will likely ease back into the water — but knows they need to stay ready if an emergency happens.

"We'll get the entire team back, and we'll pool dive, and essentially do some mental checkoffs about pool diving," Brockhouse said. "(Then) we'll be back in the water."

Jeff Alan Thumma, 43, a volunteer with the dive team, died Sunday night, according to Minnehaha County Emergency Manager Lynn DeYoung.